Resonant cavity filter temperature compensation

ABSTRACT

Resonant cavity filter having temperature compensation provided by complimentary cupped bimetallic washers positioned on opposite sides of the plunger of the resonant cavity filter. The washers support the plunger on a shaft which is adjustable within a fixed tube supported by the outer conductor or housing of the cavity. The shaft and support are threaded to provide longitudinal movement of the shaft to set the plunger to the resonant frequency, with the bimetallic washers providing temperature compensation to maintain the resonant frequency of the cavity filter.

[ June 19, 1973 United States Patent 1 1 Kommrusch 2,104,554 l/l938 Conklin 333/82 BT 2,173,908 9/1939 2,181,871

[ RESONANT CAVITY FILTER Kolster.....

TEMPERATURE COMPENSATION 12/1939 Conklin 333/82 BT [75] Inventor: Richard S. Kommrusch, Hoffman Estates Primary ExaminerRudolph V. Rol'inec Assistant Examiner-Wm. H. Punter [73] Assignee: Motorola, Inc., Franklin Park, 111.

Nov. 5, 1971 Appl. No.: 196,150

Attorney- Foorman L. Mueller, George Aichele, James W. Gillman et al.

22 Filed:

[57] ABSTRACT Resonant cavity filter having temperature compensation provided by complimentary cupped bimetallic washers positioned on opposite sides of the plunger of the resonant cavity filter. The washers support the plunger on a shaft which is adjustable within afixed tube supported by the outer conductor or housing of T4 .6 3N W 7 l 30. 31 QHB T s B 00 3/8 2 3 8 3 3 3M w 3 n .H N n. "3 m3 Wmh "c n "8 "8 Ms L Mel C mm o Umm 1]] 2 8 555 [.ll

[56] References Cited UNITED STATES'PATENTS the cavity. The shaft and support are threaded to provide longitudinal movement of the shaft to set the plunger to the resonant frequency, with the bimetallic 333/83 T washers providing temperature compensation to main- 333/82 BT tain the resonant frequency of the cavity filter.

2,472,769 6/1949 Goldstine........................ 2,439,809 4/1948 Hunter.......

3,047,818 7/1962 Regls 2,077,800 4/l937 Kroger............................

9 Claims, 5 Drawing Figures RESONANT CAVITY FILTER TEMPERATURE COMPENSATION BACKGROUND OF THE INVENTION The resonant frequency of a resonant cavity filter will vary with temperature because of the temperature coefficients of the conducting parts forming the filter. With operation in the range of very high frequencies, variance in temperature which may be encountered can cause hundreds of cycles per second variance in the resonant frequency. Wave guides have employed a number of devices to maintain the resonant frequency of the filter. These devices have included use of rods or other components formed of a nickel-steel alloy sold under the trademark INVAR. Where the variation in temperature has been slight and the accuracy required not unduly critical, the INVAR rod has performed satisfactorily. However, the cost of INVAR rods has been objectionably high to warrant a filter design which could utilize a different material.

Bimetallic structures have been utilized in tuning devices such as waveguides. However, none has provided temperature compensation for local heating of the end of the plunger forming theinner conductor of a resonant cavity filter. In those devices using bimetallic structures for compensating the tuning of oscillator circuits, none addressed the problem of local heating of the end of the plunger, which is the hottest spot in the resonant cavity when high power is applied.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a resonant cavity filter wherein the preset resonant frequency of the filter is maintained over a wide range of temperatures.

It is another object of the present invention to compensate for dimension changes due to temperature variations of a resonant cavity filter.

It is a further object of the present invention to compensate a resonant cavityfilter having high local heating due to high input power.

It is still another object of the present invention to provide a simple and inexpensive temperature compensating structure for a resonant cavity filter.

The resonant cavity filter of the invention has a conducting copper canister providing the outer support and outer conductor, and secured thereto and projecting inwardly, a tubular inner conductor having a first copper conducting can secured to its free end. A second conducting can extends about the first can and is slideable with respect thereto to form a plunger which provides variable capacitance with the end of the canister. A threaded shaft is positioned in an insert within the tube and has a slot for receiving a screwdriver to turn the same. The free end of the shaft projects through the base of the plunger, second conducting can which has contact fingers, and slideably contacts the first conducting can. Complimentary bimetallic washers mounted on opposite sides of the base of the second conducting can are indexed to the shaft and secured thereto by an E-ring. Changes in temperature, such as caused by the high power input, results in one washer cupping and the other relaxing, With increase in temperature, the length of the shaft increases and one washer cups while the other washer cups less to cause the plunger to move in opposite direction.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of the resonant cavity filter of this invention; said FIG. 2 is a perspective view in cross-section taken along lines 2-2 of FIG. 1;

FIG. 3 is a plan view of one washer;

FIG. 4 is a plan view in cross-section taken along the lines 4-4 of FIG. 3; and

FIG. 5 is a plan view in cross-section of another washer.

DETAILED DESCRIPTION Referring to FIGS. 1 and 2, the resonant cavity filter includes a copper canister 10 which provides both the outer support and also the outer conductor of the filter. The filter has an input coupling 12 and an output coupling 14, which may be provided by coupling loops or probes, as shown by my prior US. Pat. No. 3,537,041 issued Oct. 27, 1970. An inner conductor is formed by tube 16, can 18 and movable plunger or can 20 which are both mechanically and electrically coupled to the outer conductor by supports 26 and 28. The resonant frequency of the filter may be selected by movement of the plunger 20 with respect to the end 11 of outer conductor 10, with contact fingers 22 providing sliding contacts therebetween. The tube 16 is secured to the canister at the top 54 thereof by soldering to supports 26 and 28. A shaft 30 is threaded in insert 31, and supported by insert 31 and washer 33 which are secured to tube 16. The shaft 30, which may be formed from steel or the nickel-steel alloy sold under the trademark INVAR, provides a plunger support for setting the res onant frequency of the cavity filter. By rotating the shaft 30 by a screwdriver inserted into the slot 32, the plunger 20 will be longitudinally displaced along the axis of the shaft in a sliding fit with the can 18. The engagement of the threaded portion 34 of the insert 31 of tube 16 and threaded portion 36 of shaft 30 provides discrete longitudinal movement of the shaft 30 when rotated, depending upon the number of revolutions thereof, thereby tuning the filter.

The plunger 20 is indexed to the reduced cross sectional area 38 of the shaft 30 through holes 42 and 44 respectively of washer 48 and 46, and through the hole 40 of plunger 20. The washers are positioned on opposite surfaces of the plunger base 52 and secured to the shaft portion 38 with E-ring 50. The plunger 20 and can 18 together with outer conductor 10 form a transmission line, the length of which along with the spacing of the capacitor formed by the plunger base 52 and the end 11, controls the tuning.

With a high power signal, of the order of from 50 to 500 watts, applied, the inner conductor including the tube 16, shaft 30, can 18 and plunger 20 heats up. This heating is localized primarily at the base 52 or end of plunger 20, due to the high power dissipation there. Portions of the filter including the shaft 30, tube 16, can 18 and plunger 20 will change dimensions with temperature, generally expanding with increasing temperature. Accordingly, compensation is required to maintain the resonant frequency. Bimetal washers 46 and 48 control the position of the plunger 20, and thereby provide temperature compensation for the plunger. The two cupped bimetallic washers 46 and 48 compliment one another to provide sufficient movement of the plunger 20 with respect to the end 11 of the canister 10 to maintain the preset resonant frequency. These washers exert sufficient force with change in temperature to overcome the friction of the contact fingers 22 and adjust the distance from the base 52 of the plunger 20 to the end 11 of the canister 10. To provide the high degree of accuracy necessary to maintain resonance at ultra-high frequencies, relatively little compensation is necessary in the form of physical movement of the plunger.

The bimetallic washers 46 and 48 (FIGS. 2, 3, 4 and are constructed to react oppositely with changes in temperature. The height 51 (FIG. 4) of washer 46 increases with increasing temperature, while height 56 (FIG. 5) of washer 48 decreases, i.e., washer 46 cupping more and washer 48 cupping less. This complimentary effect of the two washers can be accomplished by stamping thewashers from bimetallic strips, so that with respect to the metals forming the strips, they are oppositely cupped. In one successful embodiment the bimetallic strips are of the type sold under the trademark MORFLEX with INVAR forming the inside 58 of the washer 46 and the outside 60 of washer 48, while a magnesium alloy comprising 72 percent magnesium 18 percent copper and percent nickel is positioned on the outside 64 of washer 46 and on the inside 62 of washer 48. The difference in coefficients of expansion of these two alloys will cause the two washers to react oppositely and compliment one another to move the plunger 20 with changes in temperature.

The use of such washers in the structure of FIG. 2 causes automatic movement of the plunger 20 to provide compensation for the change in dimensions produced over a relatively great range of temperatures. The proper diameter of the washers and cupping, or height, thereof has been empirically determined for a cavity 4% inches wide, 4% inches deep and 8 inches long where the plunger is 2% inches wide, 2% inches deep and 3 inches long. The heights 51 and 56 of the washer are in the range of from 0.061 inch to 0.071 inch at an average room temperature of 68F. The washers are 0.025 inch thick, have an outside diameter of 1.25 inch and a center hole diameter of 0.257 inch.

What has been described therefor is an inexpensive, but accurate means for temperature compensation of a resonant cavity filter to maintainthe preset resonant frequency thereof.

I claim:

1. In a resonant cavity filter having a support structure forming an outer conductor, and an inner conductor connected thereto, the combination including, a plunger forming a part of the inner conductor, a shaft having one end connected tothe support structure and a second end, bimetallic washer means connecting said plunger to said second end of said shaft, and means for applying signals to the cavity filter whereby the temperature of said plunger and said bimetallic washer means increases in response to the applied signals said bimetallic washer means being responsive to temperature changes to shift the position of said plunger axially alongsaid shaft, said washer means being constructed to maintain the resonant frequency of the filter substantially constant over a range of temperatures.

2. The resonant cavity filter according to claim 1 wherein said bimetallic washer means includes first and second bimetallic washers positioned on opposite sides of said plunger, said washers complementing one another to control the position of said plunger to maintain the resonant frequency of the filter.

3. The resonant cavity according to claim 2 wherein said first and second bimetallic washers comprise cupshaped structures.

4. The reasonant cavity filter according to claim 1 wherein said bimetallic washer means comprise first and second cup-shaped bimetallic washers, one of said washers cupping more with increased temperature and the other cups less. I

5. The resonant cavity filter according to claim 1 wherein said bimetallic washer means comprise washer means formed of two materials having different coefficients of expansion.

6. In a resonant cavity filter the combination including, a conducting canister forming an outer conductor having a cavity therein, a conducting tube positioned in said cavity and secured to said canister, a conducting can having an end secured to said tube and a rim spaced within said canister, a shaft extending within said tube and being movably secured thereto, a plunger having a base and a skirt positioned about said rim of said can in slideable conducting engagement therewith, said plunger, said can and said tube forming an inner conductor for the cavity, first and second bimetallic washers secured to said shaft and engaging said base of said plunger and positioning said plunger with respect to said shaft, and means for applying signals to the cavity filter whereby the temperature of said end of said plunger and said washers increases in response to the applied signals, said washers changing configuration with changes in temperature to shift the position of said plunger axially along said shaft to maintain the resonant frequency of the filter substantially constant over a range of temperatures 7. The filter of claim 6 further including a threaded sleeve secured within said conducting tube, and wherein said shaft has a threaded portion extending within said sleeve, said shaft being rotatable to change 7 the position of said plunger with respect to said tube.

8. The filter of claim 6 wherein said washers are cupped and have center openings, and said shaft extends through said openings and includes means for holding said washers thereon, aid washers extending on opposite sides of said plunger and having edges engaging said opposite sides of said plunger.

9. The filter of claim 8 wherein said washers are formed of bimetallic material in complementary configurations so that one washer cups more as the temperature increases, and the other washer cups less. 

1. In a resonant cavity filter having a support structure forming an outer conductor, and an inner conductor connected thereto, the combination including, a plunger forming a part of the inner conductor, a shaft having one end connected to the support structure and a second end, bimetallic washer means connecting said plunger to said second end of said shaft, and means for applying signals to the cavity filter whereby the temperature of said plunger and said bimetallic washer means increases in response to the applied signals said bimetallic washer means being responsive to temperature changes to shift the position of said plunger axially along said shaft, said washer means being constructed to maintain the resonant frequency of the filter substantially constant over a range of temperatures.
 2. The resonant cavity filter according to claim 1 wherein said bimetallic washer means includes first and second bimetallic washers positioned on opposite sides of said plunger, said washers complementing one another to control the position of said plunger to maintain the resonant frequency of the filter.
 3. The resonant cavity according to claim 2 wherein said first and second bimetallic washers comprise cup-shaped structures.
 4. The reasonant cavity filter according to claim 1 wherein said bimetallic washer means comprise first and second cup-shaped bimetallic washers, one of said washers cupping more with increased temperature and the other cups less.
 5. The resonant cavity filter according to claim 1 wherein said bimetallic washer means comprise washer means formed of two materials having different coefficients of expansion.
 6. In a resonant cavity filter the combination including, a conducting canister Forming an outer conductor having a cavity therein, a conducting tube positioned in said cavity and secured to said canister, a conducting can having an end secured to said tube and a rim spaced within said canister, a shaft extending within said tube and being movably secured thereto, a plunger having a base and a skirt positioned about said rim of said can in slideable conducting engagement therewith, said plunger, said can and said tube forming an inner conductor for the cavity, first and second bimetallic washers secured to said shaft and engaging said base of said plunger and positioning said plunger with respect to said shaft, and means for applying signals to the cavity filter whereby the temperature of said end of said plunger and said washers increases in response to the applied signals, said washers changing configuration with changes in temperature to shift the position of said plunger axially along said shaft to maintain the resonant frequency of the filter substantially constant over a range of temperatures
 7. The filter of claim 6 further including a threaded sleeve secured within said conducting tube, and wherein said shaft has a threaded portion extending within said sleeve, said shaft being rotatable to change the position of said plunger with respect to said tube.
 8. The filter of claim 6 wherein said washers are cupped and have center openings, and said shaft extends through said openings and includes means for holding said washers thereon, aid washers extending on opposite sides of said plunger and having edges engaging said opposite sides of said plunger.
 9. The filter of claim 8 wherein said washers are formed of bimetallic material in complementary configurations so that one washer cups more as the temperature increases, and the other washer cups less. 